Replies

1. 
   davidmeiland | Apr 16, 2007 01:42am | #1

   You could probably get away with a lightly modified like Versabond for all of that.

   The Schluter spec is for unmodified over Kerdi, but a fair number of people seem to use modified. There is a Bostik thinset called Kerdi-Set that I suppose would be lawsuit-proof.

   Your porcelain over CBU should definitely be modified.

2. 
   JTC1 | Apr 16, 2007 02:22am | #2

   	88669.1
   	>Shower Floor: 2" Slate Hexes over Kerdi ,Shower Walls: 3x6 Ceramic Subways over Kerdi, Bath floor: 1" Porcelain Mosaic Hexes over Hardibacker< Schulter downloadable installation manual suggests modified thinset under the membrane and un-modified thinset to bond the tile to the membrane. Claims the un-modified will form a stronger bond due to the impervious nature of the membrane - un-modified thinset cures better when kept damp during curing. http://www.schulter.com , Installation Handbook, page 23, "thinset facts" - I have no first hand experience with this material, however, if anything goes wrong with the installation, I have always had better luck with the manufacturers if I followed their directions. An extra $10 or $20 now for the additional un-modified thinset might pay off big later. Porcelin over HardiBacker = modified thin set - plenty of first hand experience there.  Just make sure you thinset under the HardiBacker also (fills any voids between the Hardi and the rough floor - keeps the HardiBacker rigid with no flex in relation to the floor). Luck! Jim Never underestimate the value of a sharp pencil or good light.

1. 
   jfkpdx | Apr 16, 2007 07:38am | #3

   Jim-

   Thanks for the response.  However, I found the following on the schluter web site and in one of their downloads..."The thin-set mortar used for bonding Schluter-KERDI must be appropriate for the substrate and must penetrate and engage the KERDI fleece. Generally, an unmodified thin-set mortar is used."  Were you looking at the manual for DITRA?

   I bring this up not to contradict you, but as a lead in for the question - what's the real difference between modified and unmodified thinsets?  I know the modified version has a latex polymer added, but what does it do and what's the significance?  (Psst, moderator - this might be a good topic for "What's the Difference" in the mag - if it hasn't been done already).  It seems that these days, selecting mortar is like walking down the toothpaste or shampoo aisle - there seem to be so many choices - when my gut tells me that a lot of it is just marketing.  Do all these mortars (pre-mixed, modified, Marble and Granite, unmodified, Versabond, AcrylPro, Omniwhatever) really have specific applications?  Seems like there's a ton of overlap in what each of these can do.  Tile's not my specialty, but I do like to know as much as I can about what I'm doing so thanks for setting me straight.  I guess what I want to know are there cases where using one type of mortar is an absolute no-no?

   Cheers, Jason

1. 
   davidmeiland | Apr 16, 2007 07:52am | #4

   Modified has stronger bond strengths to difficult materials like porcelain and glass, which absorb little or no moisture. Modified is always a good idea with porcelain, which makes one wonder about porcelain with Kerdi, although as has been pointed out elsewhere there are millions of porcelain installs out there with unmodified thinset.